Method of impregnating and rustproofing metal articles



United States Patent 2,990,293 METHOD OF IMPREGNATING AND RUST- PROOFING METAL ARTICLES Harry A. Toulmin, Jr., Dayton, Ohio, assignor to The Commonwealth Engineering Company of Ohio, Dayton, Ohio, a corporation of Ohio No Drawing. Filed Jan. 13, 1956, Ser. No. 558,823

' 4 Claims. (Cl. 117-50) This invention relates to an improved method of rustproofing base metals with rustproofing metals in solid, particulate state, and to the resulting products. More particularly, the invention relates to a method of protecting articles made of normally corrodible metal against corrosion without any appreciable change in the dimensions of the article by incorporating particles of corrosion-resistant metal in the pores of the metal article.

In the prior art, ferrous metals have been rustproofed by application thereto of a surface coating of rustproofing metal. Various techniques have been used to apply the coating. For example, electrochemical procedures utilizing electroplating solutions have been used widely to apply a coating of protective metal to the base metal. Such procedures are expensive, require a thoroughly clean surface on the base metal for adequate adhesion of the coating metal thereto and, moreover, involve the consideration that the successful deposition of the coating metal on the base metal depends on the relative positions of the metal in the electrochemical series.

It has been proposed, also, to apply a coating of a protective metal to a base metal by procedures involving the in vacuo decomposition of liquid or gaseous carbonyls of the coating metal. That process, conducted under vacuum, requires special apparatus comprising a closed chamher and means forevacuating air from the chamber. In addition, it requires thorough chemical cleaning of the surface of the base by electrochemical or pickling treatments prior to the coating operation.

These and other prior art methods for rustproofing the normally corrodible base metal result in the deposition of a coating which overlies the surface of the base metal object and is of varying thickness. Regardless of the thinness of the coating, inevitably it changes the initial dimensions ofthe object coated, increasing the thickness and, in the case of pieces having a circular cross-section, the diameter. I posed on the surface of the metal article are unsatisfactory when close tolerances must be observed.

The patent to Thurston, 706,701, issued August 12, 1902, proposes a method of coating a metal article with another metal by forcing particles of the coating metal, by means of a blast of a gas, against the article with suf- ,ficient force to cause the particles to become imbedded in the surface of the article and form a permanent coating thereon; The patentee states that he proposes sometimes to heat the object to be coated to open up its pores while orbefore forcing the particles of metal against it, and that the metal particles are impacted into and incorporated with the metal of the object being coated.

Extensive research shows that articles comprising ferrous metals, for example, cannot be protected against rustproofing by means of corrosion-resistant metal particles which are seemingly continuous with the article surface, by simply heating the article to open pores therein and applying metal particles of the usual available sizes against the hot surface.

Such particles tend to leave the surface and, at most, extend only partially into the very fine pores of the hot base article so that they protrude therefrom producing an irregular surface which unavoidably alters the dimensions of the article. Since some of the particles of the usual, conventional sizes projected against the surface Protective coatings which are superim- 4 ice 2 bounce off, relatively heavy bombardment of the surface with such particles is required to insure adequate protection of the metal article.

It is found, in accordance with this invention, that successful rustproofing of the metal articles without substantial change in the dimensions thereof, by means of a bombardment by a particulate corrosion-resistant metal, depends primarily on rapid absorption or adsorption of the metal particles by the base metal. This in turn, requires the use of particles of the corrosion-resistant metal having a critical diameter which is between an upper limit of 3.0 microns (sub-sieve determination) and the smallest practically obtainable size, which is about 0.5 micron (sub-sieve determination).

The term sub-sieve determination is referred to by Schmeyer and Work, in their article entitled New Methods for Particle Size Determination in the Sub-sieve Range, Am. Soc. for Testing Materials, pp. 1-22, 1941, and is universally used and accepted in metallurgy as describing metal particles below the limits of standard sieve ranges.

The present invention provides, therefore, an improved method of rustproofing articles comprising normally corrodible metals without substantial change in the dimensions thereof by impregnating the base metal particles of corrosion-resistant metal having a diameter in the critical range, i.e., between 0.5 and 3.0 microns (subsieve determination).

It is found that the metal particles of critical size are absorbed or adsorbed by, and incorporated in the base metal, even when the particles are merely laid on the surface of the metal article while the metal is in a heatexpanded or micro-porous condition, and are seemingly continuous with the base metal when it is cooled, so that the article is rustproofed without any substantial or even observable change in its dimensions.

In practice, it is convenient to heat the surface of the article to be rustproofed to a temperature below the melting point of the base metal and below the gasification or disintegration temperature for the corrosion-resistant metal and of the particulate materials and, while the surface is in the heated, microporous condition, bombard it with the particles of diameter 0.5 to 3.0 microns (subsieve determination) using any suitable device designed to project the particles against the surface under a high pressure and in a current of inert gases or a fluid such as compressed air or dry steam sufl'icient only to serve as a fluid conveying agent for the particles.

The metal particles of diameter between 0.5 and 3.0 microns (sub-sieve determination) may be particles of, for example, zirconium, titanium, zinc, aluminum, copper, magnesium or lithium. The presently preferred rustproofing particles are particles of zirconium, titanium or zinc having the critical diameter of 0.5 to 3.0 microns (sub-sieve determination).

The surface bombarded with the metal particles of the critical diameter may be in microporous condition, or a condition of maximum porosity, as a result of expansion of the metal under the influence of the heat, of the evolution from the base of residual gases such as hydrogen, entrained in the metal during a previous treatment and which, when evolved under the influence of the heat, leaves minute pores or interstices into which the particles of the corrosion-resistant metal are absorbed or adsorbed. Or the porous state of the base metal may be the result of special treatments by known processes according to which the essentially non-porous metal comprising the article or base to be rustproofed is provided with a surface layer which becomes porous on heating thereof.

In carrying out the present method, in practice, the surface of the metal article is heated in any suitable way to bring it to the microporous condition, and usually to a temperature between 500 F. and 1500? F. The heating may be accomplished using apparatus similar to conventional type drying ovens equipped with infra-red drying means capable of bringing the workpiece to a temperature of 500700 F. in about five seconds. Or the surface to be rustproofed may be brought to the desired porous condition by resistance or high frequency heating, these latter heating methods being particularly useful for large workpieces. Projection of the metal particles of diameter between 0.5 micron and 3.0 microns onto the surface to be rustproofed may be performed simultaneously with heating of the surface.

The metal particles of the diameter stated may be projected onto the base metal in an electrostatic field. Thus, the particles may be projected onto articles to be rustproofed while they are supported on a suitable conveyor moving through an electrostatic field set up between spaced, ionizing discharge electrodes and the articles, the latter serving as collecting electrodes. The metal particles are thus incorporated in and adsorbed to the articles under the influence of the field action thereon.

Incorporation of the metal particles in the article in the electrostatic fieldis especially effective because, under the influence of the field, the metalparticles areimmediately absorbed by or adsorbed to the surface.

The quantity of metal particles projected against the microporous metal surface is suflicient to provide it with absorbed and interstitially bonded corrosion resistant metal which is substantial-1y continuous with'the surface.

By the present method, employing'the combination of the three factors (a) corrosion-resistant metal particles of diameter between 0.5 and 3.0 microns sub-sieve deter mination; (b) projection of the particles against the metal surface under pressure; and (c) a heat-expanded, porous condition of the metal surface, it is possible to rustproof various metal articles very successfully after the dimensions thereof have been conformed to precise specifications to meet required close tolerances, without changing the dimensions or making complicated calculations to allow for the deposition of protective coatings which, overlying the surface of the article, increase the dimensions thereof.

Many articles made of ferrous metals and the like are given a finish or top coating of enamel or lacquer. In the past, such finish coatings have been applied on top of the coating of protective metal and then baked at elevated temperature. The present method of rustproofing the articles has the added advantage that an enamel or lacquer finish coat may be applied to the metal after the rustproofing metal articles have been incorporated in and absorbed thereby, and while the article is hot, so that the residual heat in the article assists in baking and setting the finish coat.

It has been established that the effectsobtained by the present method utilizing metal particleshaving a diameter between 0.5 and 3.0 microns, which are the result of absorption or adsorption of the particles by the base metal, cannot be obtained by bombarding the surface, in heated, porous condition, with particles of the corrosionresistant metal having a diameter substantially greater than 3.0 microns. In that case, the metal particles are not absorbed by or adsorbed to the hot porous metal and the particles penetrate the hot metal only to the extent determined by the force with which they are projected against the surface, which penetration is, at most, partial, and non-uniform. Particles of diameter less than 0.5 micron (sub-sieve determination) are not generally available. The range 0.5 micron to 3.0 microns is given to include the available particle sizes that are operative.

What is claimed is:

1. A method of treating metal articles to render the surface thereof resistant to corrosion without any appreciable change in the dimensions of the article, said method consisting of the steps of (a) heating the metal articleto a temperature between 500 and 1500 F. and

below the temperature at which said metal article will melt and high enough to open the pores providin'g'a" microporous metal surface, (b) thereafter bombarding the resultant heated microporous metal surface with metal particles consisting of a corrosion-resistant metal, said metal particles being selected from the group consisting of zirconium, titanium and zinc, said metal particles being of a diameter between 0.5 and 3.0 microns, and (c) cooling the metal :article thus treated to provide a metal article having enhanced resistance to surface corrosion.

2. A method of treating metal articles to render the surface thereof resistant to corrosion without any appreciable change in the dimensions of the article, said method consisting of the steps of (a) heating the metal article to a temperature below the temperature at which said metal article will melt and high enough to open the pores providing a microporous metal surface, said temperature being between the range of 500 F. and 1500 F., (b) thereafter bombarding the resultant heated and open microporous metal surface with metal particles consisting of a corrosion-resistant metal, said metal particles consisting of titanium metal which are of a diameter between 0.5 and 3.0 microns, and (c) cooling the metal article thus treated to provide a metal article having enhanced resistance to surface corrosion.

3. A method of treating metal articles to render the surface thereof resistant to corrosion without any appreciable change in the dimensions of the article, said method consisting of the steps of (a) heating the metal article to a temperature between 500 and 1500 F. and below the temperature at which said metal article will melt and high enough to open the pores providing a microporous metal surface, (b) thereafter bombarding the resultant heated and open microporous metal surface with metal particles consisting of a corrosion resistant metal, said metal particles consisting of zirconium metal and which are of a diameter between 0.5 and 3.0 microns, and (c) applying a baking enamel finish coat to the resultant article while the article is still hot whereby the residual heat assists in baking and setting the finish coat.

4. A method of treating metal articles to render the surface thereof resistant to corrosion without any appreciable change in the dimension of the article, said method consisting of the steps of (a) heating the metal article to a temperature below the temperature at which said metal article will melt and high enough to open the pores providing a microporous metal surface, said temperature being between 500 F. and 1500 F., (b) establishing an electrostatic field adjacent the surface of said metal article to be treated, (c) thereafter bombarding the restutant heated and open microporous metal surface with metal particles consisting of a corrosion resistant metal under the influence of said electrostatic field whereby said metal particles are absorbed into the microporous surface, said metal particles consisting of zinc metal and which are of a diameter between 0.5 and 3.0 microns, and (d) cooling the metal article thus treated to provide a metal article having enhanced resistance to surface corrosion.

References Cited in the file of this patent UNITED STATES PATENTS 553,296 Aylsworth Jan. 21, 1896 701,298 Cowpe'r-Coles June 3, 1902 706,701 Thurston Aug. 12, 1902 706,702 Thurston Aug. 12, 1902 1,014,750 Burgess Jan. 16, 1912 1,244,414 Bernheim Oct. 23, 1917 1,513,349 Stewart et a1. Oct. 28, 1924 2,149,253 Cooper Mar. 7, 1939 2,304,709 Rubin Dec. 8, 1942 2,414,923 Batcheller Jan. 28, 1947 2,442,485 Cook June 1, 1948 (Other references on following page) I 5 UNITED STATES PATENTS Parrish Nov. 18, 1952 Davis May 12, 1953 Schladitz Jan. 4, 1955 Poorman Aug. 2, 1955 Weinn'ch Ian. 17, 1956 Gfeller July 10, 1956 6 OTHER REFERENCES Johnson et al., Zinc Cementizing, Trans. Am. Electrochem. Soc. (1912) 21; 561.

Coating for Metals, by Burns and Bradley, copyright 5 1955, p. 62, published by Reinhold Publishing Corp., 430

Park Ave., New York, NY. 

1. A METHOD OF TREATING METAL ARTICLES TO RENDER THE SURFACE THEREOF RESISTANT TO CORROSION WITHOUT ANY APPRECIABLE CHANGE IN THE DIMENSIONS OF THE ARTICLE, SAID METHOD CONSISTING OF THE STEPS OF (A) HEATING THE METAL ARTICLE TO A TIMPERATURE BETWEEN 500 AND 1500*F. AND BELOW THE TEMPERATURE AT WHICH SAID METAL ARTICLE WILL MELT AND HIGH ENOUGH TO OPEN THE PORES PROVIDING A MICROPOROUS METAL SURFACE, (B) THEREAFTER BOMBARDING THE RESULTANT HEATED MICROPOROUS METAL SURFACE WITH METAL PARTICLES CONSISTING OF A CORROSION-RESISTANT METAL, SAID METAL PARTICLES BEING SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, TITANIUM AND ZINC, SAID METAL PARTICLES BEING OF A DIAMETER BETWEEN 0.5 AND 3.0 MICRONS, AND (C) COOLING THE METAL ARTICLE THUS TREATED TO PROVIDE A METAL ARTICLE HAVING ENHANCED RESISTANCE TO SURFACE CORROSION. 